The invention relates to a method of manufacturing a metal-oxide varistor with electrodes connected at the end surfaces, the energy absorption capability of which has been improved by arranging it such that the current displacement which normally arises, especially in connection with high impulse currents, close to the edges of the electrodes is avoided by increasing the resistivity of the block in the vicinity of the envelope surface. More particularly, the invention relates to a method of achieving a high-resistance zone close to the envelope surface of a metal-oxide varistor, thereby preventing the harmful effects which normally arise in connection with the above-mentioned current displacement.
Varistors comprising a body of metal-oxide powder, preferably of zinc oxide, with or without stabilizing additives and with electrodes connected at the end surfaces are used because of their nonlinear, voltage-dependent resistivity in current-limiting applications such as, for example, surge arresters. It is known that, at high impulse currents, an increased current density is obtained close to the edges of the electrodes. To avoid this current displacement, which may lead to local overheating of the varistor close to the edge of the electrode and hence to breakdown, it is known to provide the metal-oxide varistor with a high-resistance surface zone which comprises the region close to the edges of the electrodes. In this way, the current displacement is prevented and the current is distributed essentially uniformly over the electrode/varistor contact surface. The ability to be subjected to high impulse currents, without breaking down, for periods of time of the order of magnitude of 1 ms or more is referred to as energy absorption capability.
Usually, see for example German publication DE-OS 2 365 232, the high-resistance surface zone is achieved by applying a paste layer of a suitable material, for example SiO2, B2O3, Bi2O3, Sb2O3, In2O3, or mixtures thereof, onto a metal-oxide varistor, preferably a zinc-oxide varistor. Thereafter, the varistor with the applied layer is sintered again, thus obtaining a high-resistance layer with a thickness of a few tens of xcexcm. The high-resistance layer is accomplished partly by diffusion from the applied layer into the metal-oxide varistor, partly by the applied layer sintering to the metal-oxide varistor.
To ensure also a satisfactory high-current capability (impulse currents below 4-20 xcexcm), while at the same time improving the energy absorption capability, it is required, as described in xe2x80x9cIncreased Energy Absorption in ZnO Arrester Elements Through Control of Electrode Edge Marginxe2x80x9d (IEEE Transactions on Power Delivery, Vol. 15, No. 2, April 2000), that the edges of the electrodes have a certain minimum distance to the envelope surface of the varistor. This distance should be at least 0.3-0.6 mm, which means that the high-resistance layer described above is too thin in order to achieve the desired effect.
To obtain a high energy absorption capability while at the same time ensuring a satisfactory high-current capability, it is desired to achieve a considerably thicker high-resistance zone, 0-6 mm, than what is possible to achieve by applying a paste layer onto a sintered varistor body and diffusion during repeated sintering. According to an alternative method (see Swedish patent publication 466 826), such a thick high-resistance surface zone is obtained by forming a metal-oxide powder into a cylindrical body and heat-treating it at 400-600xc2x0 C. in order to obtain a porosity of 30-50%, the pores close to the envelope surface being open. The envelope surface is supplied with a metallic salt solution by spraying, dip-painting or some other equivalent method. The metallic salt solution penetrates into the pores to a depth of 2-6 mm, whereupon sintering of the varistor body with the metallic salt supplied thereto is completed at 1100-1300xc2x0 C. The alternative method thus implies dividing the sintering into two steps, which increases the manufacturing cost.
According to the invention, a metal-oxide varistor with a high-resistance surface zone of 0-6 mm and hence improved energy absorption capability is manufactured by applying a paste layer of a high-resistance material onto a pressed, but not sintered, cylindrical body of metal-oxide powder, whereupon sintering of the coated body is completed in one step. Thus, the invention eliminates the extra sintering which is required according to the prior art.
A cylindrical metal-oxide varistor is formed by pressing metal-oxide powder. The envelope surface of the cylindrical body pressed by metal-oxide powder, is coated with a paste or a dispersion of a high-resistance material, for example SiO2, LiO2 or Cr2O3 or salts thereof. The paste or the dispersion may be applied to the envelope surface of the pressed cylindrical body by dip-painting, spraying, rolling or in any other suitable way. After the coating, the coated cylindrical body is sintered at 1100-1300xc2x0 C. for 2-10 h. During the sintering, the high-resistance material penetrates by diffusion into the surface zone of the envelope surface. The depth of penetration and the amount of absorbed high-resistance material, which controls the resistivity in the surface layer, depend on the composition of the paste, the thickness of the paste, the microstructure of the cylindrical body, the sintering temperature and the sintering time.